Process for the manufacture of products of beryllium or beryllium alloy



Aug. 1, 1967 .MALLEN ET AL 3,333,994 3 PROCESS FOR THE MANUFACTURE OFPRODUCTS OF BERYLLIUM' OR BERYLLIUM ALLOY Filed Oct. '6, 1964- 0,2impel-afar:

United States Patent 3,333,994 PROCESS FOR THE MANUFACTURE OF PRODUCTSOF BERYLLIUM OR BERYL- LIUM ALLOY Jos Mallen, Paris and Michel Weisz,Orsay, France, as-

signors to Commissariat a IEnergie Atomique, Paris, France Filed Oct. 6,1964, Ser. No. 401,862 Claims priority, application France, Oct. 25,1963, 951,895 7 Claims. (Cl. 14811.5)

The present invention relates to improvements in processes for themanufacture of beryllium products which are intended notably foraviation, astronautics and atomic energy.

It concerns more particularly processes of this type which are concernedwith the production of beryllium tubes, and more particularly tubessuitable for the casing of fuel elements of nuclear reactors.

Since the beginning of the utilisation of beryllium in aircraft and thenin space craft, and finally as a nuclear material, for which fields itis particularly suitable by virtue of its remarkable physicalproperties, the desired beryllium products have always been made frombillets or blooms of sintered beryllium. Onlysome parts of simple shape,such as sheets, have been obtainable from cast metal, and even then ithas been necessary to effect a grain refinement in order that thismaterial may have acceptable mechanical characteristics.

Endeavours have therefore been made to improve the ductility ofberyllium tubes and for this reason recourse has hitherto been had topowder metallurgy for the production of such tubes, since the sinteredmetal has a much finer grain than the cast metal, as also much higherlongitudinal ductility (in the direction of the cold working at lowtemperature).

However, despite many efforts to increase the threedimensionalductility, tubes of this type have proved incapable of meeting certainrequirements in regard to casings for nuclear fuels.

Of the many stresses which are set up in the course of the operation ofthe reactor, the casing of a fuel pencil, i.e. of a rod of very smalldiameter in relation to its length, must notably withstand with anappropriate safety margin those which correspond to the thermal stressesresulting from its association with the fuel (each of these members hasits own programme temperature and its own coefficient of expansion).

During continuous operation, the casing is at a temperature which isdetermined by its position, the neutron balance of the reactor, thespeed of the cooling fluid, etc., and it is normally sufiiciently hot toafford relief from any stresses.

In transient operation, particularly during cooling, the situation ismore critical owing to the thermal stresses set up in the casing. Inthis respect, the entry temperature of the fluid, usually 200-250 C., isof great importance.

It has been found that with a casing, the major problem is to obtainsufiicient transverse ductility at low temperature in order to avoidrupture.

Tubes obtained by powder metallurgy do not possess such ductility in asufficient degree. Efforts have therefore been directed to theproduction of tubes possessing this essential form of ductility and ithas been discovered that, to some extent, the longitudinal ductility andthe grain size are only of secondary importance.

The present invention therefore has the object of providing a processfor the preparation of products of beryllium or beryllium alloy andparticularly tubes, which more fully satisfies than hitherto the variouspractical re- 3,333,994 Patented Aug. 1, 1967 quirements, and inparticular permits of obtaining a less fragile product of morefavourable texture which is not detrimentally affected by maintenancefor long periods at temperatures of the order of 700 C., which is muchcleaner from the viewpoint of inclusions, exhibits few intergranularflaws and above all possesses distinctly higher transverse ductility.

The invention resides mainly in that-while an ingot of cast beryllium orberyllium alloy, optionally subjected to a compression, is used asstarting material-the said ingot is subjected, in the course of itsshaping to at least one cold working operation, optionally followed by atepid drawing, a straightening by stress relief with imparteddeformation at a temperature close to that of the recrystallisation ofthe metal, and followed by a controlled recrystallisation annealing.

The invention further comprises, apart from this main feature, certainother features which are preferably used 'at the same time, and whichare to be considered separately or in any technically possiblecombinations:

The said cold working operation or operations consist in impactextrusion, the metal being optionally coated with a thin layer ofsilver,

The said cold working operation or operations consist in extrusion undera casing, which casing may notably consist of soft steel,

The said cold working operation or operations consist in a pre-extrusionunder a thin casing followed by an extrusion under a thick casing,

The extent of the extrusion is at least equal to 50.

In any case, the invention will be more readily understood from thefollowing further description and the following example, which are, ofcourse, given only by way of example. In the course of this description,reference will be made to the single figure which gives, as a functionof temperature, the transverse ductility of the beryllium treated by theprocess according to the invention 'and of sintered beryllium.

The use of cast metal as starting material, instead of sintered metal asusual, is one of the most important features of the present inventionbecause it shows that cold-worked cast metal is substantially free fromhot fragility, that its texture is more favourable than that ofcold-worked sintered metal, that maintenance at 600- 700 C. for longperiods is beneficial to the cold-worked cast metal, while it is harmfulto cold-worked sintered metal, that inclusion radiographs have revealedhigher cleanness of the cold-worked cast metal, and finally that,although its presentation and its surface appearance seem to be lessgood, it is less subject to defects such as intergranular microcraoks,which are harmful to transverse ductility in the case of tubes.

The following table shows a comparison of the longitudinal andtransverse ductilities of sintered and cast beryllium ingots:

Minimum temperature for an elongation of 10% Longitudinal Transverse(DL), 0. (Dr), Q o.

Extruded sintered sheet bar 50 400 Extruded cast sheet bar 250 extrusionor the conventional extrusion under a casing may be used, but the secondmethod gives greater lengths of more uniform tubes, as will hereinafterbe seen with reference to the following example.

The tubes of cold-worked cast metal according to the present inventionmay also be subjected to a tepid drawing which improves the tolerancesof the surface condition, and gives greater pliability in thecontrollled recrystallisation treatments. Thus, drawing passes of to at450 C. are possible without intermediate annealing.

The tubes thus prepared are thereafter subjected to stress relieffollowed by annealing at a temperature close to the recrystallisationtemperature, i.e. between 550 and 750 C., which process improves theelastic limit at elevated temperature and the ductility of theberyllium.

There will therefore be given in the following an example of theproduction of beryllium tubes by the process according to the presentinvention.

There is used as starting material a cast beryllium ingot which iscompressed for 2 minutes at 650 C. under a pressure of 10,000 bars, thiscompression having the object of eliminating any remaining small pipesand thus providing a clean ingot.

In the course of a pro-extrusion at 950 C., a billet in the form of athin soft steel casing 2 mm. thick is coldworked to a degree or an areareduction of 6.

The bar thus obtained is cut into sections and pierced to supply thebillets intended for the extrusion of tubes.

A second extrusion is thereafter performed at 950 C. at a speed of 27mm./second and to a cold-working degree or an area reduction of 15.

The trueing of the tubes is thereafter effected by deformation impartedin tension, in the course of which a considerable stress relief occursat 650 C. in half an hour, and annealed at 750 C. for 4 hours. As statedin the foregoing, the product obtained is more ductile and less fragilethan an equivalent product prepared from sintered metal, as is shown bycomparison of the curves A and B of the single figure.

Curve A corresponds to a beryllium tube obtained by the process of theinvention. Curve B corresponds to a sintered tube. Dr% is the percentageof elongation at the moment of rupture of a test bar subject totraction.

Of course, the invention is in no way limited to the mode of applicationjust referred to, or to the embodiment described, but covers allvariants thereof, and notably the manufacture of all beryllium productsfrom cast metal,

not only for nuclear uses, but also for requirements of aviation andastronautics.

What is claimed is:

1. A method for manufacturing tubes of beryllium and beryllium allloycomprising the steps of first working a cast ingot of beryllium andberyllium alloy at about 950 C. with an area reduction of about 6, thenfurther working said ingot at about 950 C. with an area of reduction ofabout 15, then subjecting the drawn ingot to stress relief with plasticdeformation in tension at a temperature approaching recrystallizationtemperature of the metal in the range of 550750 C. and then annealing ata temperature of about 750 C. for about four hours.

2. A method as described in claim 1, including the step of firstcompressing the cast ingot of berylflium and berryllium alloy at about650 C.

3. A method as described in claim 1, including the step, after thefurther working step, of then tepid drawing said ingot at about 450 C.

4. A method as described in claim 1, said working steps comprisingimpact extrusion, the ingot being coated with a thin layer of silver.

5. A method as described in claim 1, said working steps consisting ofextrusion under a casing of soft steel.

6. A method as described in claim 1, said first working step consistingof extrusion under a thin casing followed by said further working stepof extrusion step under a thick casing.

7. A method as described in claim 1, said working steps consisting ofhot extrusion of the ingot with an area reduction at least equal to :1.

References Cited UNITED STATES PATENTS 2,872,363 '2/1959 Macherey14811.5 3,065,117 11/1962 Brown et al. 14813 3,234,052 2/1966 Wikle148l1.5

FOREIGN PATENTS 868,064 5/1961 Great Britain.

884,108 12/1961 Great Britain.

884,410 12/1961 Great Britain.

919,423 2/1963 Great Britain.

DAVID L. RECK, Primary Examiner.

H. F. SAITO, Assistant Examiner.

1. A METHOD FOR MANUFACTURING TUBES OF BERYLLIUM AND BERYLLIUM ALLOYCOMPRISIG THE STEPS OF FIRST WORKING A CAST INGOT OF BERYLLIUM ANDBERYLLIUM ALLOY AT ABOUT 950*C. WITH AN AREA REDUCTION OF ABOUT 6, THENFURTHER WORKING SAID INGOT AT ABOUT 950*C. WITH AN AREA OF REDUCTION OFABOUT 15, THEN SUBJECTING THE DRAWN INGOT TO STRESS RELIEF WITH PLASTICDEFORMATION IN TENSION AT A TEMPERATURE APPROACHING RECRYSTALLIZATIONTEMPERATURE OF THE METAL IN THE RANGE OF 550-750*C. AND THEN ANNEALINGAT A TEMPERATURE OF ABOUT 750*C. FOR ABOUT FOUR HOURS.